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How often is a person’s biological father someone other than the man they call dad?

Paternity doubts are a source of gossip, on the one hand, and emotional trauma, on the other. But our assumptions about infidelity may be based more on the headlines of salacious magazines than on hard facts.

Who’s your Daddy?, a provocatively titled documentary airing on SBS at 8:30pm Sunday April 20, delves into rates of what is variously know as non-paternity, “misattributed paternity” or “disputed fatherhood”.

Genetic testing for paternity has transformed such disputes from subjectively pitting the credibility of the mother and the alleged father against each other, to a more objective process where the natural variation in each person’s DNA is used to determine the likelihood of a genetic relationship.

With such precise science available, you might imagine the rate of misattributed paternity would be quite easily measured, but this is not the case. So the documentary sets out to determine what the actual rate is.

Dubious details

The film sets the scene by describing an unpublished study of blood groups of parents and children of between 200 and 300 families in the United Kingdom, done “possibly in the 1950s” in southeast England.

It reportedly showed 30% of children had blood groups different from their putative father, suggesting the men didn’t really father the children.

The study was testing not for paternity but for antibody formation, according to the researcher Dr Elliott Elias Philipp, who quoted the figures at a symposium. And it leaves many unanswered questions about the nature of the population studied and the quality and robustness of the data.

Nevertheless, this 30% figure became frequently quoted as the general rate of non-paternity and is used throughout the documentary as the “shock! horror!” baseline to which comparison are made.

To elucidate the current rate of non-paternity, the documentary presents results from a world-first study of 2,200 Australian and American women, who chose to do an online survey.

You might imagine the rate of misattributed paternity is easily measured with DNA testing but this is not the case.SBS

Three questions were posed:

have you had consecutive male partners within a month?

have you had any sex outside of a long-term relationship?

have you ever conceived when married from another relationship?

The answers generate interesting information about patterns of sexual behaviour among the participants, but the documentary doesn’t tell us some key information: who are the participants in the survey? Are they representative of the general population or mainly very literate, well-educated people with high socioeconomic backgrounds fooling around on the internet?

The documentary makers missed a real opportunity here. Think about it, if you’d been unfaithful around the time of the conception of your child, would you admit it in an online survey?

Without this information, the survey generates curious but uninformative data, and we’re left no wiser about the underlying question of the rate of non-paternity in the general community.

Mix and match

The documentary is a curious mix of attempts at genuine measurement of non-paternity and pseudoscience on theories of human behaviour, based on anthropomorphising people comparing their sexual behaviour and monogamy practices with those of swans and fairy wrens.

This is its weakest aspect – where speculative theories are presented as probable fact – and it diminishes what is mostly a genuine effort to understand what the science can and can’t tell us.

DNA fingerprinting carried out for entirely different reasons – testing carried out in the military services, for instance, or for medical or forensic reasons – may actually provide some of the best evidence we have of true non-paternity rates. And these data show vastly lower rates than 30%.

With over 15 years of experience in the clinical genetics setting, we have observed very low rates of accidental discovery of non-paternity: fewer than five cases among the hundreds of families we see clinically.

But of course, we don’t claim our experience is necessarily representative, and families where non-paternity is an issue might avoid genetic testing for that very reason.

Perhaps the single greatest contribution of this documentary is to question the dogma around the 30% non-paternity rate figure. While its attempts to get better answers lack rigour, it starts a conversation we need to have – because at the centre of these sometimes titillating stories is a mother and a child and a father.